Kepler was instrumental in replacing our Geocentric – Earth centered – view of the Universe with a heliocentric – Sun centered – view. This heliocentric view would later fall. Two major mileposts on our way towards a modern view of our Universe.

Kepler’s three laws are stated in our times thus:First Law:
The orbit of every planet is an ellipse with the sun at a focus.Second Law:
A line joining a planet and the sun sweeps out equal areas during equal intervals of time.Third Law:
The square of the orbital period of a planet is directly proportional to the third power of the semi-major axis of its orbit. Moreover, the constant of proportionality has the same value for all planets.

The first law describes the shape of a planet orbit. The second law tells us that a planet moves faster when closer to the Sun than when further away. The third law tells us that the length of a planet’s year depends on its distance from the Sun. The further out a planet orbits the Sun; the longer its year is.

Isaac Newton was later able to derive these laws using his Theory of Gravitation.

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Did you notice that a Scientific Law describes how natural phenomenon behave?

A Scientific Theory on the other hand explains why things follow the laws they are observed to follow.

The scientific use of the words law and theory differ from the common use of these words. In common use a Law is a rule agreed upon by common agreement and a theory is an untested idea that has not been proved. In common use a theory is weaker than a law. In science the opposite is true.

Note that scientific theories are never proved to be true. Theories can either be proved false (rare) or (more commonly) replaced by a new theory that explains natural phenomenon even better. Newton’s Laws of Gravitation have not been replaced by Einstein’s General Theory of relativity. Relativity is just more accurate than Newton’s Theory in extreme situations. When sending a probe to the Moon; Newton’s Theory of Gravitation works just fine. Sending a probe into orbit round a black hole on the other hand would require use of Einstein’s General Theory of Relativity because the velocities in question are much closer to the speed of light and therefore more extreme.